That's exactly why putting a number or letter value on a game just doesn't work - it just leads to irrelevant comparisons.
I think the main reason a "buy it", "rent it", "avoid it" scale hasn't been put to practice is because the PR companies for these games would flip a shit. Let's be honest here - reviews and review sites are essentially advertisements disguised as information. To bluntly say "avoid this game" or "just rent this game", even if that's what a mediocre letter/number score may imply, would really hurt the game's marketing campaign.
That said, I also forgot to mention the fact that not only am I a fan of the 3-tiered rating system, I am also a fan of multiple people reviewing a single game. EGM does (did?) this, but very few other sites do. I dislike the fact that many people on forums just tie a review to a particular site - "IGN gave this game a 9!" or "Gamespot gave it a 7.8!" when it should be "Hillary Goldstein gave this game a 9" and "Jeff Gerstman gave this game a 7.8." If you ignore the "money-hat, bias conspiracy," these reviews are just one person's opinion. If you have differing preferences than this person, or value things differently in a game than them, then your "score" would be different. This is why having multiple people on a single review is ideal.
But that requires time and money, which most review sites just don't have. And in EGM's case, they sacrifice review depth/content when they have 3 people reviewing 1 game (note, I haven't read an EGM in a while, so this may have changed).